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Old 01-19-2016, 11:25 AM   #1
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Question on Solar Panel output voltage

I am testing a portable 90W Bosch solar panel (actually three 30W wired together). The rated output is 17.5V. I am working in full sun, but it is January, and I'm near Boise. The panel is tilted near 90% to the sun.

With the panel disconnected from the controller (which is attached to the panel), I get 20+ volts (didn't write down the exact amount).

When I connect the controller to a 12v battery (the battery is at about 80% charge), I only measure 14.5V +/- going INTO the charge controller from the solar panel. I thought I should see 17.5V, since that is what the panel is rated at.

My question is, what volts should I see from a 17.5V panel before it goes into the controller, when the controller is connected to a battery? Does the low winter sun have an effect on the voltage output from the panel (when the panel is near 90% to the sun)?

Thanks- Bob
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Old 01-19-2016, 11:41 AM   #2
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All your numbers look fine to me. The open circuit voltage of 20+ volts is pretty normal. The charge voltage of 14.5 is proper through the charge controller for regular flooded cell batteries. The difference between the 17 + volts you mention and the charge voltage to your batteries shows that the charge controller is working.

All is well.
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Old 01-19-2016, 01:15 PM   #3
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[QUOTE=idroba;1738236]All your numbers look fine to me. The open circuit voltage of 20+ volts is pretty normal. The charge voltage of 14.5 is proper through the charge controller for regular flooded cell batteries. The difference between the 17 + volts you mention and the charge voltage to your batteries shows that the charge controller is working.


I don't think I explained this very well. On the input side of the charge controller (the voltage between the solar panel and the charge controller, not between the charge controller and the battery), I am only getting about 14.5 volts.

Shouldn't I be getting around 17.5 volts to the charge controller? (Especially since it is mounted on the back of the solar panel so there should be very little voltage drop in that short distance.)
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Old 01-19-2016, 01:47 PM   #4
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The input voltage will be dragged down by the load of the controller and batteries.

With a partly discharged battery, and a good sun at 90 degrees to the panel, your 90 watt system should charge at about 4.5 to 5 amps in my estimation. Measure that current flow, it is more important than any voltage measurement.

No, you will not get 90 watts from your panels, in general I figure only about 75% of the rated panel number will actually be delivered to the batteries and be useful to charge. This is real world experience.
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Old 01-19-2016, 01:48 PM   #5
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Quality panels will show you output voltage going in to the controller at values approximating their STC ratings. When I was using a 500 watt array of GS-100 panels, I would see voltages in the 18 VDC area on a regular basis.

When I switched to flex panels last year as an experiment, that voltage dropped dramatically to between 14.5-15.0VDC. Same 500 watt array but now comprised of flex panels.

The difference in the amount of charging amps is staggering..... Up to 35 amps to the batteries with hard panels vs. 24-25 amps with the flex array.

The difference is solely due to the lower output voltage of the flex panels, which also provide little to no solar charging boost thru the Blue Sky 3024iL controller at the batteries.


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Old 01-19-2016, 02:14 PM   #6
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Lewster: "Quality panels will show you output voltage going in to the controller at values approximating their STC ratings. When I was using a 500 watt array of GS-100 panels, I would see voltages in the 18 VDC area on a regular basis."

Would the input voltage change with the sun lower in the winter or higher in the summer? If so, approx how much?
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Old 01-19-2016, 02:28 PM   #7
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Idroba: "The input voltage will be dragged down by the load of the controller and batteries."

I wondered if that would have an affect on the input voltage.


Idroba: "With a partly discharged battery, and a good sun at 90 degrees to the panel, your 90 watt system should charge at about 4.5 to 5 amps in my estimation.

I am getting 5+ amps (on a Trimetric) when I hook the portable unit to my camper, using a 30' 10ga extension cable and Anderson connectors. And I'm getting this whether I run from the solar panel directly to the camper's controller, or if I use the solar panel controller and then run to the camper batteries. It is, however, slightly higher if I run directly to the camper's PWM controller.

I guess my concern is if I bypass the controller on the panel, and run straight to the controller on my camper or trailer, I don't want excessive voltage drop. If I start with 17v at the panel, I should have 16+ when it gets to the controller on the trailer or camper, with the extension cable. But since I'm getting 5+ amps from a 90W panel in January, maybe I'm overthinking it...
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Old 01-19-2016, 07:03 PM   #8
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You do not want to have two charge controllers in the circuit. The one in the trailer is the one which you should be using. It is closest to the batteries. Charge controllers for 12 volt nominal systems, in general, are completely happy with input voltages in the 17 volt range.
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Old 01-19-2016, 08:30 PM   #9
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Quote:
Originally Posted by Trailbob View Post
With the panel disconnected from the controller (which is attached to the panel), I get 20+ volts (didn't write down the exact amount).

When I connect the controller to a 12v battery (the battery is at about 80% charge), I only measure 14.5V +/- going INTO the charge controller from the solar panel. I thought I should see 17.5V, since that is what the panel is rated at.
The output voltage of a solar panel depends on the load current; if the current is very high (i.e. if it's tied to a very low impedance load) then the output voltage will drop. A maximum-power-point-tracking charge controller will monitor the current and voltage at the panel connections and adjust itself so as to pull the highest achievable power for the given situation. PWM controllers will simply pull current such that the output voltage becomes equal to the desired charging voltage - this may pull the panel output down to the point where the available power is less than optimal.
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Old 01-19-2016, 09:17 PM   #10
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I like solar power too.
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Old 01-24-2016, 09:11 PM   #11
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24 Volt Solar Panels

Since the topic is about solar panel voltage, what would be the disadvantage of using 24 volt (nominal) solar panels connected to a dual voltage 12/24 MPPT Charge Controller (Blue Sky)?
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Old 01-24-2016, 10:59 PM   #12
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Advantages: lower current for the same power, so you might get away with using your existing pre-wiring!

Disadvantages: high cost for the MPPT controller.
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Old 01-24-2016, 11:16 PM   #13
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You do have to be very careful of the maximum voltage that the MPPT can accept. Many, including the older Blue Sky controllers can run on the ragged edge to well over their range with some of the higher voltage panels, especially when used in series.

The higher voltage panels are also usually physically larger and thus can be harder to find space for on the trailer roof.
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